The present invention relates to a method of and a system for controlling the ratio between the water flow rate in the primary circuit and the water flow rate in the secondary circuit of a hot water heating system, such as, without limitation, a low-temperature heating system, providing for return water passing through an adjustable bypass to be mixed with flow water passing through a multi-way mixing device.
If conventional devices not incorporating a bypass are used for controlling the ratio between the primary and secondary circuit flow rates in a low-temperature heating system, such as, without limitation, an underfloor heating system, the maximum flow temperature of 50.degree. C. is already obtained when the mixing device is in a mid-range position, assuming an average boiler water temperature of 80.degree. C., thereby losing the benefit of some 50% of the mixing device control range and accepting inaccurate and hence unsatisfactory control characteristics.
If a preset bypass is incorporated, substantially the entire rangeability of the mixing device is exploited, thereby achieving reasonably accurate control characteristics. Known methods of and systems for so obtaining reasonably accurate control characteristics provide for the bypass butterfly valve to be set to an appropriate position prior to the use of the heating system, thereby fixing said position for the entire mixing device control range. The additional quantity of return water fed into the flow piping is sufficient for the maximum flow water temperature not to be exceeded, if and when the way from the primary circuit through the mixing device to the secondary circuit is fully opened.
Said known method of and equipment for controlling the ratio between the water flow rates in the primary and secondary circuits using a preset bypass are associated with a number of disadvantages encountered during the operation of the heating system. Flow control is, for example, very slow particularly as the flow temperature approaches the maximum flow temperature, since the additional quantity of return water mixed with the flow water through the bypass is comparatively large. Steady state conditions at maximum flow temperature or near maximum flow temperature are only reached after many hours or never reached. Said disadvantages of conventional control systems only allow a very slow increase in the heat supplied to the space heated during the heating-up period after the night-time temperature reduction even if modern heating system controllers allow a much more rapid heat input.
In the cases of variable heat output boiler plants and/or boilers no longer oversized for economic or financial considerations which no longer feature any extra capacity to increase the room temperature from a low level to a normal level under design temperature conditions, it is therefore no longer possible to incorporate a non-adjustable bypass between the flow and the return piping.